Anti-caking agent for ammonium nitrate



limited States Patent Mi ANT I-CAKIN G AGENT FOR AMMONIUM NITRATE Paul0. Marti, Jr., Munster, Ind., assignor to Standard Oil Company, Chicago,111., a corporation of Indiana No Drawing. Application December 31, 1952Serial No. 329,146

11 Claims. (Cl. 23-403) This invention relates to a particulate ammoniumnitrate composition which is substantially non-caking and substantiallyfree-flowing at atmospheric humidity. Also, the invention relates to animproved method of grinding ammonium nitrate in the presence ofatmospheric humidity.

Ammonium nitrate is hygroscopic. In the presence of atmospheric humidityparticulate ammonium nitrate takes up moisture and the individualparticles tend to agglomerate to form lumps; under conditions of highhumidity the ammonium nitrate particles tend to form a solid mass. Thisagglomeration tendency of ammonium nitrate in the presence ofatmospheric humidity is commonly called caking or setting.

For most commercial uses it is desirable that the ammonium nitrateparticles be free-flowing, e.g., when used as fertilizer, it isnecessary that the granules flow readily from the hopper through thedistributing slots onto the ground. For use in the explosive industrythe powdered ammonium nitrate should flow readily from the storage binsinto the mixing vessel. Bagged ammonium nitrate granules tend to cakeand this cake must be broken up before use.

Unless considerable care is taken to maintain low atmospheric humidity,it is extremely difiicult to obtain powdered ammonium nitrate of aparticle size suitable for explosive use. Even with care regardingatmospheric humidity, it is at present a slow and expensive grindingoperation to prepare finely powdered ammonium nitrate.

It is an object of this invention to prepare a particulate ammoniumnitrate composition that is not susceptible to atmospheric humidity.Another object of the invention is a composition consisting of ammoniumnitrate particles coated with a defined anticaking-agent. Still anotherobject is a composition consisting of ammonium nitrate particles coatedwith a comminuted agent, which composition is characterized byessentially free-fiowability at atmospheric humidity and by a storagestability substantially that of dynamite grade ammonium nitrate. Aparticular object of the invention is a method of grinding ammoniumnitrate in the presence of atmospheric humidity to produce powderedammonium nitrate more quickly and more cheaply than by the prior artmethods. Another particular object is an improved method of grindingammonium nitrate in the presence of atmospheric Patented Aug. 25, 1959The term ammonium nitrate as used in this specification and in theclaims is intended to include either ordinary commercial grade ammoniumnitrate or dynamite grade ammonium nitrate or pure ammonium nitrate.

The term atmospheric humidity is intended to mean the presence of watervapor in air at various temperatures; or even the atmospheric conditioncommonly known as fog. More particularly, the term is intended toinclude those conditions at which the water vapor present in the airwould cause agglomeration and caking of ammonium nitrate particles.

The copper oxide anticaking-agents of this invention may be eitheranhydrous or may contain small amounts of adsorbed water. Either thenaturally occurrying material or synthetic material may be used. Thepreferred individual members are: Cupric oxide, CuO; cuprous oxide, CuO.

The anticaking-agents (grinding aids) of this invention must have aparticle size markedly smaller than the particle size of the ammoniumnitrate which is to be treated. In order to obtain substantiallycomplete coating a comminuted mixture of the agent is preferred.Especially effective agents have a particle size between about 0.01 andabout 10 microns; as much as 10 weight percent of particles having anaverage size somewhat greater than 10 microns may also be present. It ispreferred to use agents having a particle size mainly below about 1micron when the agent is intended for use solely as an anticaking-agent.

The anticaking-agent appears to operate by forming an adherent coatingon the outer surface of the ammonium nitrate particle. This coating ofagent substantially preeludes the water vapor in the atmosphere fromcontacting the ammonium nitrate. Furthermore, the agent particles reducethe agglomerating tendencies of the ammonium nitrate particles so thatthe flow characteristics of the particulate coated-ammonium nitrate isbetter than that of the uncoated particles even in a dry atmosphere. Thecoating slows, somewhat, the rate of solution of the nitrate in liquidwater; however, the solubility in liquid water is not changed.

The anticaking-agents of this invention adhere readily to the ammoniumnitrate particles. A non'caking composition is easily obtained by simplemixing of ammonium nitrate particles and a suitable amount of theanticakingagent. More finely divided coated ammonium nitrate particlesmay be obtained by passing simultaneously ammonium nitrate particles anda suitable amount of antibe desirable to regrind the product from thefirst grinding operation.

Some improvement in caking characteristics can be obtained by theaddition of small amounts of agent. For best results and maximumimprovement in caking characteristics at least sufficient agent shouldbe added to the ammonium nitrate particles to substantially coat theentire outer surface of the ammonium nitrate particles. Amounts of agentin excess of this quantity have no significant beneficial effect on thefree-fiowability of the coated ammonium nitrate particles. The amount ofanticaking-agent needed to coat a given quantity of ammonium nitrateparticles is dependent upon the size of the ammonium nitrate particles.The smaller the particles the more agent needed per unit weight ofammonium nitrate. For fertilizer size grains as little as 02 weightpercent, based on ammonium nitrate, of cupric oxide or cuprous oxide issufficient. For the ammonium nitrate particles in so-called dynamitegrade, between about 0.5 and 1.0 weight percent of cupric oxide orcuprous oxide may be needed. For the fine powder desirable for someexplosive usages wherein most of the particles pass through a 325 meshscreen as much as 4 or 5 weight percent of cupric oxide or cuprous oxidemay be needed. In general the amount of anticaking-agent needed toobtain a particulate ammonium nitrate composition characterizedbyessentially free-fiowability at atmospheric humidity is between about0.2 and 5 weight percent of comminuted anticaking-agent wherein thelarger amount corresponds to the finer averageparticle size of theammonium nitrate.

The anticaking-agents of this invention catalyze the decomposition ofammonium nitrate. The presence of copper oxide in ammonium nitrate canresult in an explosion when the ammonium nitrate is heated to about 130C. The higher the purity of the ammonium nitrate, the more catalystrequired. The presence of copper 0X- ide markedly increases the rate ofdecomposition, as measured by gas evolution, at higher atmospherictemperatures. However, in general, the amounts of anticakingagentsneeded to produce free-flowing ammonium nitrate particles at atmospherichumidity are sufiiciently less than the amounts needed to catalyze thedecomposition of ammonium nitrate that the coated particles arecharacterized by storage stability substantially that of dynamite gradeammonium nitrate. Dynamite grade ammonium nitrate contains a sufficientamount of impurities such that the stability is not as great as pureammonium nitrate; nevertheless, the stability of the dynamite grade issufficient for all explosive purposes.

In order to illustrate the results obtainable with the anticaking-agentsof this invention, the following illustrative examples are set forth:

Example I Screen mesh number: Wt. percent retained A quantity of thisammonium nitrate was placed in an 8-ounce screw-top jar. The top wasloosely screwed on so that the atmosphere would have access to theinterior of the jar. The jar was placed on a shelf in a room; after 7days exposure to room temperatures between about 70 and 85 F. andrelative humidities of between about 30 and 70%, the ammonium nitrateparticles had caked not only together, but had adhered to the glasssurface. It was possible to invert the jar without any of the ammoniumnitrate particles becoming detached from the mass or the mass becomingdetached from the bottom of the jar.

Rim 2.This was carried out by mixing about 1 weight percent ofcommercial grade cupric oxide, based on ammonium nitrate, with the sameamount of ammonium nitrate particles as used in run 1. The mixing wasaccomplished by stirring the oxide and the ammonium nitrate particleswith a spatula.

An electron microscopic examination of the oxide showed that theparticle size ranged from 0.01 to about 10 microns; most of theparticles were less than 1 micron 1n slze.

peratures of between 70 and F. and relative humidi ties between about 20and 80%.

Run 3.A test was made with cuprous oxide under the same conditions asrun 2. The results with the cuprous oxide appeared to be identical withthose of the cupric oxide.

Example II Run 4.Dynamite grade ammonium nitrate was ground in aMikro-Pulverizer at a speed of 9600 rpm. using a slotted screen at arelative humidity of 70%. The first ground material was reground underthe same conditions to maximize the finer particles. screen analysis ofthis material was:

Screen mesh number: Wt. percent retained +80 0.3 +120 4.7 +200 37.0 +32535.0 325 23.0

A quantity of this 9600 rpm. material was placed in an 8-ounce glass jaras described in run 1. After 2 days exposure to room temperature ofabout 75 F. and a relative humidity between about 50 and 70%, thematerial was caked solid and adhered firmly to the glass jar. The jarcould be inverted without detaching any of the caked material.

Run 5.About 3 weight percent of cupric oxide (same as in run 2) wasadded to a quantity of the 9600 rpm. material. Microscopic examinationof the mixture showed the ammonium nitrate particles to be completelycoated with a layer of the agent. After 2 months exposure to the sameatmospheric conditions as those given in run 2 the coated ammoniumnitrate was still substantially free-flowing, although some very smalllumps had formed.

Run 6.This run used cuprous oxide as the agent. Under the conditions ofrun 5, it appears that cuprous oxide and cupric oxide are equallyeffective as anticakingagents. 2

Example III Run 7.-Dynamite grade ammonium nitrate was ground in theMikro-Pulverizer at 9600 r.p.m. The ground material was ground a secondtime. The room temperature during the grinding was about 75 F. and therelative humidity was about The ground material tended to blind thescreen. The total grinding time for 200 grams of ammonium nitratecharged was somewhat more than one-half hour.

The 9600 rpm. material formed lumps in a short time and in about 2 dayswas caked solid when exposed in a loosely capped jar to a roomtemperature of about 75 F. and a relative humidity of about 70-80%.

The Rotap screen analysis of this 9600 r.p.m. material made immediatelyafter grinding-was:

Screen mesh number: Wt. percent retained Run 8.-Under the sameatmospheric conditions as those given in run 6, 200 grams of dynamitegrade ammonium nitrate and 6 grams of cupric oxide were charged to theMikro-Pulverizer operating at 9600 rpm. The.

ammonium nitrate and the oxide were commingled by The Rotap shaking in ajar before being charged to the hopper. The total time for two passesthrough the pulverizer in this run was about 5 minutes. The Rotapanalysis of the coated ammonium nitrate product from this run was:

This run illustrates the remarkable reduction in grinding time obtainedby the use of comminuted copper oxide as a grinding aid and thesimultaneous considerable decrease in average particle size of theproduct ammonium nitrate. It is obvious that a considerable decrease inspeed of the Mikro-Pulverizer can be made when using grinding aid inorder to obtain the same particle size distribution as that given in run6.

Electron microscopic examination of the product ammonium nitrateparticles showedthe ammonium nitrate to be substantially completelycoated with a layer of oxide. The coated ammonium nitrate particlesshowed only a few very small lumps after about 2 months exposure to theconditions of run 2.

Thus having described the invention, what is claimed is:

1. A particulate composition consisting of ammonium nitrate particlescoated with a comminuted copper oxide, which coated particles arecharacterized by substantially free-flowability at atmospheric humidityand by a storage stability substantially that of dynamite grade ammoniumnitrate.

2. The coated particles of claim 1 wherein said oxide is cupric oxide.

3. The coated particles of claim 1 wherein said oxide is cuprous oxide.

4. A particulate composition consisting of ammonium nitrate particlescoated with between about 0.2 and 5 weight percent, based on ammoniumnitrate, of a copper oxide, having a particle size between about 0.01and 4 microns, where the smaller percentage corresponds to largerammonium nitrate particles, and which composition is characterized byfree-flowability at atmospheric humidity and by a storage stabilitysubstantially that of dynamite grade ammonium nitrate.

5. The composition of claim 4 wherein said oxide is selected from theclass consisting of cupric oxide and cuprous oxide.

6. An improved method of grinding ammonium nitrate in the presence ofatmospheric humidity, which method consists of grinding ammonium nitrateparticles in the presence of a comminuted copper oxide to obtain nitrateparticles of size smaller than those charged.

7. The method of claim 6 wherein the particle size of said oxide isbetween about 0.01 and 10 microns.

8. The method of claim 6 wherein said oxide is cupric oxide.

9. The method of claim 6 wherein said oxide is present in an amount atleast sufiicient to substantially coat the surface of the productammonium nitrate particles.

10. An improved method of grinding ammonium nitrate particles in thepresence of atmospheric humidity, which method consists of grindingammonium nitrate particles in the presence of between about 0.3 and 5weight percent, based on the nitrate, wherein the larger amountcorresponds to the finger average particle size of the ammonium nitrate,of copper oxide, having a particle size between about 0.01 and 10microns, which product nitrate particles are of a smaller size than theparticles charged.

11. A non-caking ammonium nitrate composition which consists of ammoniumnitrate particles having a Rotap screen analysis: +14 mesh, about 3 wt.percent; +30 mesh, about wt. percent; +80 mesh, about 12 wt. percent;and --80 mesh, about 5 wt. percent; coated with about 1 wt. percent,based on nitrate, of cupric oxide, having a particle size between about0.01 and 10 microns, mainly below about 1 micron.

Harned Oct. 2, 1883 Cairns et a1 Mar. 3, 1942

1. A PARTICULATE COMPOSITION CONSISTING OF AMMONIUM NITRATE PARTICLESCOATED WITH A COMMINUTED COPPER OXIDE, WHICH COATED PARTICELS ARECHARACTERIZED BY SUBSTANTIALLY FREE-FLOWABILITY AT ATMOSPHERE HUMIDITYAND BY A STORAGE STABILITY SUBSTANTIALLY THAT OF DYNAMITE GRADE AMMONIUMNITRATE.